The invention relates to Fabry Perot sensors generally and more specifically to such sensors in which the sensed parameter acts upon the sensor by changing the optical path length of a Fabry Perot cavity.
Interferometer instrumentation is known in the prior art for measurement of optical phase shift, precise wavelength, very small distances, thickness and for other known applications. In the present instance, the measurement of optical, magnetically-induced phase shift is contemplated using Fabry Perot interferometer concepts in a novel arrangement. The change can be detected by analyzing interference effects resulting from placing a first cavity optically in series with a second Fabry Perot cavity. For those interference effects to occur it is necessary for the two cavities to be of similar optical path length; that is to say the difference in their optical path length must be smaller than the coherence length of the light used to observe the interference.
Each cavity behaves much as a comb filter whose "teeth" represent narrow pass spaced apart in frequency (wavelength) by an amount dependent upon cavity length. The sharpness of the "teeth" depends upon the finesse of the cavity, which is a measure of its Q (quality factor) and is determined bgy the reflectivity of its end facets, the absorption losses associated with the medium between the facets and losses due to facet misalignment. If the optical path length of the first cavity differs from that of the second by a small integral number of half-wavelengths, the "teeth" of one comb will register (be congruent) with those of the other, with the result that light transmitted through one of the cavities will suffer little attenuation on passing through the second. Thus high transmission is associated with phase relationships of 0.degree. and 180.degree.. If however, the optical path length of either cavity changes by a small amount so that the phase relationship moves away from 0.degree. or 180.degree., then the "teeth" of one comb become displaced from registry with those of the other, and the transmission rapidly falls away.
By arranging the relative optical path lengths of the two cavities such that the transmission of the combination is approximately 0.7 times its maximum value, a regime is established in which very small changes of relative phase produce relatively large changes in transmission. In principle, a particular change in transmission can be correlated with a particular change in phase, but this requires knowledge of the precise shape of the transfer function relating the two parameters. Hence, in a Fabry Perot sensor system, it is instead generally preferable to use a feedback system to maintain the relative phase of the two cavities by driving a stretcher acting on the optical path length of one of the cavities with an error signal derived from a detector measuring the transmission of the cavity combination.
The classical Fabry Perot cavity consists of two spaced apart plane parallel reflectors. For some applications the space between the reflecting facets contains air, while for certain critical work vacuum has been preferred so that the operation of the device will not be influenced by refractive index changes induced by changes of ambient temperature. The present invention concerns optical waveguide cavities in which the transmission medium between the reflecting facets is provided by an optical waveguiding structure which may, for instance, be an optical fiber of a waveguide channel defined in a slab of dielectric material. A device employing that type of waveguide cavity is described, for example, by S. J. Petuchowski et al. in an article entitled "A Sensitive Fiber-Optic Fabry Perot Interferometer" appearing in the IEEE Journal of Quantum Electronics, Vol. QE-17 No. 11, November, 1981 pp 2168-70.
An advantage of a waveguide cavity over the bulk optic equivalent is that the stringent requirement for the facets to be plane and parallel to avoid beam "walk-off" is greatly relaxed. This is because the light reflected from the facet will still be guided as long as it falls within the acceptance angle of the waveguide.
In addition, the waveguide cavity is inherently more robust as the facets are an integral part of the structure, and hence cannot be misaligned, which makes it particularly suited to sensor applications in hostile environments. With this type of cavity, the optical path length is dependent upon temperature, and hence there is a requirement either to ensure quite stringent temperature control or to take steps to quantify and compensate for errors resulting from temperature effects. The present invention is particularly concerned with the latter approach, and employs a reference cavity thermally strapped to the sensor cavity for the evaluation of temperature induced effects.